<div [ngClass]="themeObj">
  <h4>请选择您喜欢的主题</h4>
  <button (click)="changeTheme('blueTheme')">湛蓝天空</button>
  <button (click)="changeTheme('greenTheme')">青青草原</button>
  <button (click)="changeTheme('darkTheme')">暗黑系列</button>
  <p>loremng elit.trum sint aut sequi odio quae. Culpa, beatae illum et inci</p>
  <p>Lorem ipsum dolor sit amet consectetur adipisicidunt laudantium odit expedita quae. Inventore, sed?</p>
  <p> Nesciunt dignissimos magnam repudiandae consequuntur nos</p>
</div>